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Old 26th March 2007, 09:19 AM   #1
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Default Trouble in a new DIY amp.

I've built a PP EL34 following the Lundal/Byrith document, but used hashimoto 40-5 transformer and bulit the of PSUs different.

I can turn it on with dummy load attached and measure the DC - levels thrugh both channels. They are fine, and so are the heather voltages. Tubes also seemes fine. Bias is -28 at grid, 460mV over the catode res. Some hum in one channel, the other is very silent.

Connected a CD-player and a pre to it yesterday. One channel played fine a while, the other was still silent. Only a faint hum was audible. Tried switching input connections, but when I turned it on again the secondary fuse (500mA, just before B+ tap in the HT-psu) was blown.

I have some pdf and powerpoint schematics I`ve drawn, but I do not know how to upload them. I do also get a lot of help on a norwegian forum. I`ll post if we find some faults.
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Old 26th March 2007, 09:40 AM   #2
singa is offline singa  Singapore
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Hi,
Just guessing, since one channel does not work,there may be a connection/wiring problem.The fuse may be rated too low if the transformer is a large one.TRy 1A onwards.
Cheers.
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Old 26th March 2007, 10:15 AM   #3
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There is a 2A fuse on the primary of the xformer. The fuse in the B+ was supposed to be 300mA, but only I had a 500mA availible.
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Old 26th March 2007, 10:22 AM   #4
singa is offline singa  Singapore
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Hi,
Better recheck wiring of defective channel .Good luck.
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Old 26th March 2007, 05:08 PM   #5
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A paticularly trickey place to check is the coupling between the driver stage and the output stage, make sure you are getting good levels of resistance, Make sure the cap isn't leaking DC, and make sure the coupling stage isn't shorting anything to ground.
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Old 27th March 2007, 12:14 PM   #6
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Found a wire break to the grid of one EF86, so that explained the lack of signal. For the fuse, C filter takes a while to reach max, so when the cathodes are hot (fuse popped when I turned on - of - and on again) the fuse pops. I'll rebulid a delay circuit so it does not encompass the C filter, only B filter.
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Old 28th March 2007, 04:36 PM   #7
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any new updates? I am now intrigued
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Old 28th March 2007, 06:12 PM   #8
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Still working on it.
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Old 28th March 2007, 11:14 PM   #9
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What are you using for power transformers? Are the toroids by any chance? Toroids, by nature of the remanence of the core, will sometimes blow a fuse when powered up. It has to do with the phase of the AC when turned off, then the phase when turned on again. I once had it figured out what set of conditions result in trouble, but my memory... It doesn't matter...

I have also had the problem with seriously oversized EI transformers, though I have not figured out why.

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Old 29th March 2007, 08:47 AM   #10
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Its not a toroid, but a dynaco - replacement thing from triodeelectronics. Seems to be running quite cool and with low hum, buy the way.
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